Biodiversity
Biodiversity includes the diversity of genes, populations, species, communities, and ecosystems. This diversity can be measured by variety (the number of different types), quantity and quality (what types are there and how many of each one), and by the spatial distribution (are they everywhere or just at a single spot).
Genetic diversity
Biodiversity at the within-species level is usually measured by gentic diversity, which refers to the variety of alleles and allele combinations. For example, there may be several alleles of the genes controlling the colour of a blossom. One version might result in blue petals, while the other version results in red petals.
Genetic diversity can enhance the resistance of a species population towards disturbances and environmental changes.
Species diversity
As indicated above, species diversity encompasses not only species richness (the number of species), but also the questions, which functional groups of species are present and how abundant are they.
Ecosystem diversity
An ecosystem is a natural unit consisting of all biotic factors in an area together with all abiotic factors of the environment (i.e. tropical rainforest, coral reefs, savannas). Different ecosystems consist of different functional groups of species and deliver a diversity of roles to the human being.
For further reading, see for example Chapter 4 of the Millenium Ecosystem Assessment.
